Cafeity Get the app

Living with it

Muscle loss on a GLP-1

Any large weight loss takes some lean mass with it, and GLP-1 medicines are no exception. The body-composition scans from the trials give a clearer picture than the headlines: most of what is lost is fat, a meaningful share is lean, and the two things with the best evidence for protecting muscle are protein and resistance training.

By the Cafeity teamUpdated 4 min read

The short answer

On GLP-1 medicines most weight lost is fat, but some is lean mass. In the SURMOUNT-1 body-composition substudy, people on tirzepatide lost 21.3% of body weight at 72 weeks, including 33.9% of fat mass and 10.9% of lean mass (placebo: 5.3%, 8.2% and 2.6%).1 Across studies the lean share of weight lost varies widely — from about 15% or less to 40–60% — and a 2024 review concluded that, on MRI, muscle changes look largely in line with the amount of weight lost.2 The 2025 joint advisory recommends adequate protein — 1.2 to 1.6 g per kg of body weight a day has been proposed during active weight loss — and regular resistance training.3

Bar chart from the SURMOUNT-1 body-composition substudy at 72 weeks on tirzepatide: body weight down 21.3% (placebo 5.3%), fat mass down 33.9% (placebo 8.2%), lean mass down 10.9% (placebo 2.6%).
Average change from baseline in the SURMOUNT-1 DXA substudy (160 participants), tirzepatide doses pooled. Lean mass includes water and organs as well as muscle.

What the scans show

The labels for semaglutide and tirzepatide both say the medicine lowers body weight "with greater fat mass loss than lean mass loss".45 The best numbers come from DXA scans done within the trials. In SURMOUNT-1, a substudy of 160 participants found tirzepatide reduced fat mass by 33.9% and lean mass by 10.9% over 72 weeks, and the proportion of weight lost as fat or lean was broadly consistent across subgroups.1

Other studies report very different proportions. A 2024 review found that lean mass has made up anywhere from about 15% or less of total weight lost in some GLP-1 studies to 40–60% in others, depending on the population, the medicine and other conditions.2 It also stressed two cautions: lean mass on a scan includes organs, bone, fluid and water in fat tissue as well as muscle; and in MRI-based studies, reductions in muscle volume appeared broadly in line with what would be expected for age, disease and the weight lost, with muscle quality often improving.2 Older age and more severe illness raise the stakes, which is why the review flags sarcopenia risk in choosing who these medicines suit.

Why it still matters

Muscle matters for strength, balance, blood sugar control and — as people age — independence. The 2025 joint advisory makes preserving lean mass one of its priorities during GLP-1 treatment.3 Bone is part of the same picture: in the semaglutide cardiovascular outcomes trial, more hip and pelvis fractures were reported on semaglutide than on placebo in women (1% against 0.2%) and in people aged 75 or over (2.4% against 0.6%).4

Protein

The advisory notes that 1.2 to 1.6 g of protein per kg of body weight per day has been proposed during active weight loss (or 1.2 to 2.0 g per kg of adjusted body weight during major weight loss), notes that a practical daily figure of 80 to 120 g can be easier to follow, and warns that intake should not drop below about 0.4–0.5 g per kg.3 Cafeity's own reference figure is 1.4 g per kg — see the protein calculator and the printable protein food list. People with kidney disease should agree a figure with their doctor.

Strength training

Protein alone is not enough; the advisory is explicit that resistance training is needed alongside it, and that aerobic exercise alone does less to preserve lean mass during rapid weight loss. It recommends regular resistance training together with at least 150 minutes a week of moderate aerobic activity.3 Bodyweight exercises, bands and weights all count; what matters is working the major muscle groups regularly and progressively. If you are new to exercise, have a heart condition or have been unwell, check with your doctor first.

Keeping a record of it

If your scale writes body-fat or lean-mass readings to Apple Health, Cafeity can show them next to your weight. Protein sits on the Today screen against your target, so the days it falls short — often the days after a dose — are visible.

Cafeity Today screen: day 1 of 7 since the last dose with the estimated level at 97% of peak, fibre 21 of 25 g, protein 110 of 100 g, steps, water 850 ml of 2.4 L, the next logged dose and the weight trend.
Protein against your target.
Cafeity weight screen showing a smoothed trend weight of 115.6 kg, down 5.5 kg since last week, with the note "Based on your recent rate. Not a prediction."
Weight as a smoothed trend.

Download Cafeity on the App Store

Common questions

How much muscle do you lose on a GLP-1?
In the SURMOUNT-1 scans, lean mass fell by 10.9% while fat fell by 33.9% on tirzepatide over 72 weeks.1 Across other studies the lean share of weight lost varies widely, and lean mass is not the same as muscle.2
How can I prevent muscle loss on a GLP-1?
The 2025 joint advisory recommends adequate protein and regular resistance training, alongside aerobic activity.3
How much protein should I eat to keep muscle?
The advisory notes that 1.2 to 1.6 g per kg of body weight a day has been proposed during active weight loss, or roughly 80 to 120 g a day as a practical figure.3 Agree your own figure with a clinician.

Sources

  1. Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study Look M et al. Diabetes, Obesity and Metabolism 2025
  2. Changes in lean body mass with glucagon-like peptide-1-based therapies and mitigation strategies Neeland IJ, Linge J, Birkenfeld AL. Diabetes, Obesity and Metabolism 2024
  3. Nutritional priorities to support GLP-1 therapy for obesity: a joint advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association and The Obesity Society Mozaffarian D et al. 2025 (published in four journals)
  4. Semaglutide injection and tablets for weight management — US prescribing information DailyMed, US National Library of Medicine, version published 30 June 2026
  5. Tirzepatide injection for weight management and obstructive sleep apnoea — US prescribing information DailyMed, US National Library of Medicine, version published 2 September 2026

Written by the Cafeity team from the sources listed here. Figures from prescribing information are quoted as the label states them; trial results are averages from those trials, not predictions for any one person. This page has not yet been reviewed by a clinician — see the editorial policy for how pages are researched and corrected.